The present disclosure relates to industrial ink jet printing systems and related methods of operation. More particularly, it relates to controlled delivery of ink to a piezoelectric print engine component of industrial ink jet printing systems.
Industrial ink jet printing systems are widely employed across many industries to generate printed indicia on a multitude of products and packaging. For example, ink jet printing systems are commonly used for printing text, bar codes, and graphics on consumer products, building materials, and packaging, all on a mass production basis. The printing systems are often in-line with the manufacturing and/or packaging process, and print real time information directly on the articles of interest. The types of information typically printed include production date, expiration date, lot and shift codes, bar codes, company graphics, product name and description, etc.
Many industrial ink jet printing systems utilize a piezoelectric drop on demand print engine (“piezoelectric DOD printing”) to dispense ink for printing. In general terms, piezoelectric DOD print engines incorporate an ink-filled chamber (otherwise connected to a reservoir of ink) made all, or in part, from a piezoelectric material that is positioned behind a nozzle plate. When printing is desired, a voltage is applied to the piezoelectric material behind nozzle(s) of the nozzle plate. This voltage causes the piezoelectric material to change shape, in turn generating a pressure pulse in the ink to force a droplet of ink from the nozzle(s). Once the droplet is dispensed, the voltage is removed, the piezoelectric material returns to its original shape, and the chamber draws ink from a separate reservoir to refill. This cycle is repeated to generate successive droplets of ink.
As part of the above-described printing operations, the print engine acts like a pump by using ink and nozzle plate surface tension properties together with physical changes or pulses from the piezoelectric material. With this construction, however, there is no structural impediment to ink flow straight through the print head. To generate necessary control over ink flow to the head, then, a slight vacuum must be maintained on the ink delivered to the print engine to prevent ink from flowing through freely, while surface tension between the ink and the nozzle plate holds ink in the nozzles against the force of the vacuum. This delicate balance between ink vacuum and surface tension must be maintained for consistent printing performance in piezoelectric DOD printing.
Conventionally, one of two techniques is incorporated into the piezoelectric DOD ink jet printing system to create the necessary vacuum while delivering ink to the print engine. With a passive approach or system, gravity assists in creating the desired vacuum. A pool of ink, open to the atmosphere, is positioned slightly below the print engine in a local reservoir. An ink line is run from this local reservoir “up” to the print engine. Pressure is temporarily applied to the local reservoir to force ink through the ink line and to the print engine, displacing the air in the system (“priming” the system). Once the system is primed, the weight of the ink in the ink line pulls (via gravity) on the surface tension of the ink in the nozzles and establishes a vacuum in the print head. The height of the local reservoir relative to the print engine is adjusted to create the optimum vacuum level at the print engine during use. As ink is dispensed from the head during printing, it is replaced by ink from the local reservoir. In very basic passive systems, the ink level in the local reservoir is maintained manually, while higher end systems automate the process of replenishing the local reservoir. While gravity-based ink systems can provide the necessary vacuum/surface tension balance, for many end users these systems are not viable. In particular, components of the passive system (e.g., the local reservoir) are oftentimes in the way of the product to be printed on, or the packaging line, due to the requirement of the ink pool needing to be below the print engine.
An alternative piezoelectric DOD printing system design actively maintains a vacuum in the head through the use of a vacuum pump and control circuitry. These “active”-type systems typically use a header tank that is closed to atmosphere and positioned many inches above the print engine. An ink line leads from the header tank to the print engine. A vacuum is applied to the pool of ink in the header tank and appropriately controlled to hold the ink in the tank, and to deliver ink to the print head at the ideal vacuum level. As compared to passive system, active systems are larger, much more complex, and more expensive.
In addition to the above problems, existing industrial piezoelectric DOD printing systems often require a great deal of user oversight, for example with set-up, calibration, and maintenance. Along these lines, while some existing piezoelectric DOD print systems allow for printing in both horizontal and down “shooter” orientations (i.e., arrangement of the print engine relative to horizontal) and all angles in between, significant user efforts are required to recalibrate the system before the printing orientation can be changed.
In light of the above, a need exist for a compact piezoelectric DOD printing system with controlled delivery of ink to the print engine that overcomes the deficiencies of existing designs.